The present invention relates to a method and apparatus for treating pulp. A problem resulting in the development of the method and apparatus according to the invention becomes especially obvious during the feed and discharge of blow tanks used in connection with batch digesters. Thus, putting it more precisely, the method and apparatus according to the invention relate to the filling and discharging of various pulp tanks and storage containers.
It is known from prior art that a so-called blow tank is used in connection with batch cooking departments. Contrary to continuous cooking, batch cooking is understood to be a pulp production method in which the cooking department comprises several digesters, most commonly 5-10 in one cooking department. Each of said digesters in turn is filled with chips and cooking chemicals which are allowed to effect the chips for a certain period of time, after which the digester is discharged to a so-called blow tank. The timing of the feedings and discharges of the digesters is effected so that the blow tank is filled at fairly regular intervals. The blow tank is meant to function as an intermediate storage, blow container, of the cooked pulp, from which the pulp is discharged in an even flow to the next, invariably continuously operating process.
A problem discovered in operating said blow tanks is that the consistency of pulp being discharged from the blow tank does not remain within ranges required by the following process stage, i.e. the brown stock washing department. A reason for this is that each digester has its own way of discharging depending on e.g. the operator, the uniformity and execution of the cook in general as well as the raw wood material and pulp quality. During the discharge process, the consistency of the pulp may vary even between zero and ten percent. A reason for this consistency variation is that in the beginning of the discharge process practically only liquor is discharged from the digester, because after the so-called final displacement which in modern batch cooking processes is the last process prior to the discharge of the digester, there is a certain amount of displacement liquor in the bottom part of the digester. This is naturally discharged from the digester first, before the actual pulp. During the discharge, and more intensively towards the end of the discharge, the pulp has to be diluted in order to make it flow from the digester to the discharge pump. Just at the final stage of the discharge, the pulp has to be diluted even more, as the hydrostatic pressure under which the pulp is discharged from the tank is substantially lower than in the beginning of the discharge due to low height of the pulp column.
In the blow tank, the consistency variation often results in channeling of the pulp so that at various locations of the blow tank the consistency of the pulp alters to a large extent. Although the blow tank is provided with one or several mixers, our experience has shown that these are not capable of equalizing the consistency of the pulp in all conditions, but the pulp is discharged from the tank to the following process stage at an unsuitable consistency. In the worst case, the consistency of the pulp is decreased in the mixing zone of the tank far below the desired values.
A second problem consists in filtration of the pulp to form a cake on the surface of the pulp existing in the blow tank. A direct reason for this is the lack of motion in the upper part of the mixing zone and the fact that the pulp being discharged from the digester always contains air and gases originating from the cook which are not easily removed from the pulp. This phenomenon is also dependent on the quality and type of pulp. Especially when running the tank in order to empty it or sometimes even in normal running situations, depending on the surface level, said filtrated pulp causes remarkable consistency variations when entering the mixing zone at the bottom part of the tank, which consistency variations can not be equalized in the mixing zone, but the pulp is discharged further at too high a consistency.
FIG. 1 illustrates the consistency variation of pulp during the blow of one digester. As already stated earlier, the consistency of the pulp varies to a great extent during the discharge, being low both in the beginning and in the end due to reasons stated before. In the middle stage of the blow the consistency of the pulp may be in the order of ten percent. The time interval between two blows is in the order of 20-40 minutes depending on the size and number of digesters. In our tests we have noticed that said interval of about half an hour is sufficient to cause the pulp on the surface in the blow tank to filtrate, i.e. thicken, whereby onto the surface of the pulp existing in the tank a relatively solid, and continuously solidifying, cake is formed.
FIG. 2a illustrates a case in which the digester is discharged to a blow tank via a conduit arranged at the upper end of the tank. In this case it is obvious that the pulp is discharged, if no auxiliary means distributing the flow are used, with considerably great force deep into the pulp already existing in the tank. Thus, in the one hand, the pulp filtered on the surface in the blow tank cannot at any stage enter the mixing zone, except if the tank is being run to empty it or to lower the surface level, but the pulp starts to be collected on the surface in the tank. At the worst case the pulp may start to deteriorate, if the hardened pulp cake remains in the tank for a long period of time. On the other hand, the dilute pulp discharged deep into the pulp existing in the tank forms a local agglomeration, which at some stage when the blow tank is being evenly discharged is suddenly discharged into the mixing zone decreasing the consistency of pulp being discharged from the tank to below the desired values.
FI patents 98836 and 100011 deal partly with the same problem and suggest as a solution various apparatuses by means of which it is possible to somewhat restrict the downward flow of pulp entering the mixing zone or on the other hand fill the blow tank better than before.
Said better filling of the blow tank is illustrated in FIG. 2b where it can be seen how the apparatus according to FI patent 100011 is used to feed pulp into the tank via the bottom part of the tank to a certain altitude. Although said solution does help in many problems related to the filling of the tank, FIG. 2b illustrates also a situation where dilute pulp is being discharged to the tank through the apparatus. This channels directly from the inlet opening of the feeding apparatus towards the mixing zone, resulting in principle in the same kind of situation as the solution according to FIG. 2a. 
Naturally, when treating pulp, problems of the same kind appear not only in connection with blow tanks but elsewhere as well, e.g. in connection with pulp storage tanks at various stages of the pulp treatment process.